Evaluate the effect of the administration of a single IV dose of BCG cell wall vaccine on the extent and rate of development of lymphosarcoma in cattle infected with bovine leukemia virus at a stage prior to the development of discernible evidence of disease. Correlations will be sought between immunologic status, as monitored by in vitro tests, and the course of disease. Supplies of vaccine and dose specification will be provided by the Government. Included will be a minimum of 31 pairs of age and risk matched animals, one member of each pair to receive vaccine and the other to act as untreated control Animals will be assigned to treatment or control populations on a random basis.